From the Moscow Times:
There are two reasons why Putin rushed to change the Constitution only six months after stepping down as president. First, he sees the political and economic dangers of falling oil prices. The house of cards built on an eight-year oil boom is crumbling. Second, Putin understands that as the crisis develops, there could easily be a fierce battle among opportunistic politicians and businessmen to seize troubled assets.
Putin must act now before it is too late. In only six months, a rival group could be formed as an alternative to his siloviki to take advantage of the public discontent and power vacuum caused by the crisis. If this group becomes powerful enough, it could even rally around Medvedev and convince him to dismiss the prime minister based on the government’s failures in handling the crisis.
This threat may seem farfetched, but Putin cannot completely dismiss it. When oil was more than $100 per barrel, the Medvedev-Putin duo could get away with its ersatz, or “sovereign,” democracy. But during a financial crisis, it will be much more difficult to keep pulling the wool over the people’s eyes.